Method of electroplating



United States Patent 3,396,092 METHOD OF ELECTROPLATING Robert 'C.Moyer, Garfield Heights, and Charles D. Stricker, Shaker Heights, Ohio,assignors to United States Steel Corporation, a corporation of DelawareNo Drawing. Filed June 7, 1965, Ser. No. 462,079 7 Claims. (Cl. 204-19)This invention relates to electroplating. More particularly, theinvention relates to a method of electroplating a partially,metal-coated workpiece.

A well known expedient for improving the surface properties of articles,particularly metal articles is to apply a protective coating to thesurface of the article which possesses properties superior to the baseto which it is applied. Such products as galvanized steel, tin plate,etc., are well known examples of the use of protective metal coatings toimprove the surface properties of steel. However, if for some reason itis necessary to re-coat a portion of the workpiece from which thecoating has been removed or is inadequate, care must be exercised toprovide a sound coating to the uncoated portion without harming thecoating already on the work. The problem of re-coating frequently ariseswith coated metal articles which have been welded and wherein thewelding operation exposes the base metal by removal of the protectivecoating. Similarly, metal-coated strands of wire, strip, etc. which areto be joined together into coi-l lengths are usually welded. The weldingoperation removes coating from the surface of the metal at the jointthereby exposing uncoated metal, and it is desirable to re-coat thissection to restore the protective coating.

Among coating techniques, electroplating is regarded as providing themost uniform and desirable coating, and accordingly, would be thepreferred approach for re-coating, if it were practical. However, if -apartially coated workpiece, such as the welded joint of the coatedstrands referred to above, is electrolytically coated, i.e.electroplated, in an acid electrolyte, the metal coating already on thestrand may be injured. Before the plating potential is applied, thecoating on the work may either be removed by electrolyte entirely or ifnot removed, it may begin dissolving and a deposit will form on thesurface which will not permit the desired adherence of the plating tothe core. For this reason, re-coating practices have typically avoidedelectroplating, relying instead on other methods of applying metalcoatings such as painting and immersion in molten metal. Unfortunately,attempts at re-coating the joint by applying a protective coating of hotmolten metal to the joint have proved unsatisfactory because of thenon-uniformity of such a coating and the difiiculty in controllingcoating thickness. Moreover, such coatings do not possess the desiredductility, adherence and tenacity. A method of coating the jointelectrolytically without injuring the coating already on the wire andwhich provides a uniform ductile coating of controlled thickness has,therefore, long been desired.

The present invention provides a method of electroplating such partiallycoated metal workpieces which avoids the aforementioned difiiculties.The method according to the invention permits the uncoated portion to beelectroplated without damage to the coating already present.

According to the invention, a partially plated workpiece iselectroplated as the cathode in an acid electrolyte containing a salt ofthe metal to be plated. However, prior to contacting the partiallycoated metal workpiece with the electrolyte, a holding voltage isapplied to the workpiece which is above the decomposition potential ofthe electrolyte. After application of the holding voltage, the workpieceis immersed in electrolyte to plate 3,396,092 Patented Aug. 6, 1968 it.Upon completion of electroplating, the holding voltage is maintained onthe workpiece substantially until the workpiece is withdrawn from theelectrolyte; that is to say, until withdrawing the workpiece from theelectrolyte in the electroplating cell. The term decomposition potentialas used herein refers to the voltage required to cause a sustainedcurrent flow through the electrolyte from an anode to the cathodicworkpiece. If a partially, metal-coated workpiece is inserted in an acidelectrolyte without a holding voltage previously applied thereto, theacid electrolyte will attack the coating on the work and will eitherdissolve the coating or create undesirable films or deposits whichinterfere with plating. By applying a holding voltage to acathodically-connected, partially coated work prior to contacting itwith the electrolyte, current immediately flows between the anode andthe workpiece. Thus, when the workpiece is inserted into the acidelectrolyte, the electrolyte is not able to attack the coating on thework, and/or form undesirable deposits.

An additional advantage of our method of electroplating is that it canbe readily adapted to an automatic plating system which does not requireconstant attention by an operator. Thus, for example, a system can beemployed in which a holding voltage potential is applied to the workprior to immersion in electrolyte, after which the voltage can beincreased to increase the current density and thereby increase theelectroplating rate. A timer may be included in this system to controlthe plating cycle and, upon completion of the plating cycle, to reducethe operating voltage to the holding voltage. The holding voltage canthen be maintained on the workpiece until it is withdrawn from theelectrolyte.

When electroplating a partially coated workpiece, it has been discoveredthat it is particularly desirable to vibrate the work during plating.The vibrating action action permits the plating to be carried out at ahigher current density, minimizes pinholes and greatly improves adhesionof the coating to the base metal. By using high current densities, thework can be re-plated very rapidly. Vibrating the work duringelectroplating additionally prevents coating tree-formation which occurswhen plating at high current densities onto rough surfaces by standardplating techniques. Accordingly, the preferred embodiment of theinvention contemplates vibration of the workpiece during electroplating.

The amount of the holding voltage applied to the work depends upon theparticular electrolyte employed and the electroplating conditions used.The electrolyte decomposition potential is affected slightly by thefollowing: (1) the electrolyte concentration, (2) electrolyte agitation,(3) surface condition of the work and type of electrode material and,(4) electrolyte temperature.

The theoretical electrolyte decomposition potentials for variouselectrolytes are available in textbooks and handbooks. For example, thetextbook entitled Principles of Physical Chemistry by S. H. Maron and C.F. Prutton, 3rd edition, contains on page 575, a list of decompositionpotentials of 1 N solutions at room temperature using platinumelectrodes. Some of these are:

Volts NiCl 1.85 CuSO, 1.49 CdSO, 2.03 ZnSO, 2.55

When plating the respective trolyte of the plating-metal salt, it hasbeen found that to prevent the electrolyte from attacking the partiallycoated cathodic workpiece, 0.2 to 0.5 volt should be added to thetheoretical decomposition potentials at standard conditions to insure asustained flow of current to the cathode. If the holding voltage is lessthan 0.2 volt more than the decompositionpotential, it may not besuflicient to provide a current flow between the anode and cathode. If acorrection of more than 0.5 volt is added to the decompositionpotential, the plating rate increases too much to control platingthickness without supervision. As an illustration, where a partiallyzinc-coated steel article is to be re-plated with zinc, the holdingvoltage applied, to the coated steel work would be between 2.75 to 3.05volts. This holding voltage is arrived at by adding 0.2 to 0.5 volt tothe decomposition. potential for zinc sulfate of 2.55 volts.

As a further illustration, the welded joint of galvanized wire fromwhich the zinc coating had been removed, can be re-coated byelectroplating as follows:

Zinc-coated steel wire strands, e.g. 0.06 to 0.18-inch in diameter, areprepared for electroplating by removing oxides and soils from thesurface in any conventional manner such as with the use of a wire brush.If oils or greases are present, additional cleaning with detergents orsolvent may be required. After cleaning and removal metals froma id. Q.Ih ihighacid con e ra o is de r t n e the electrolyte conductivity andto permit the use of high current densities. The electrolyte temperatureis desirably maintained between 80 and 110 F.

The wire strand is electrically connected so that it is cathodic to theanode;.within theelectrolyte. A holding voltagepotentialof between 2.8and 3.0 volts is applied to the steel Wire strand-before insertionorsuspension in the electrolyte. The applied holding voltage very slightlyexceeds" the decomposition"potential and some plating onto the cathodicworkpiece will occur. However, to accomplish the full re-coating withina reasonably rapid time, the voltage is increased to above 6 volts,preferably 6 to 7 /2 volts. In this way, current densities of from 600to 1400 amp/sq. ft. are achieved and an adherent coating ofapproximately 1 to 1 /2 mils of zinc can be deposited in one minute;When the plating cycle has been completed, the strand can be removedfrom the electrolyte and the holding voltage potential can bediscontinued.

' As discussed above, the holding voltage potential must be applied tothe workpiece before contact with the acid electrolyte. Should theworkpiece be immersed into the of detergents or solvents, if used, thewire is ready for plating.

To enable high current densities to be used and to improve the qualityof the coating, the wires should be vibrated during electroplating. Anysuitable vibrator may be used. The wires are clamped and mounted to thevibrator work-holder and the vibrator holder is connected to thenegative side of a rectifier. A DC. rectifier is used which has twovoltage positions: one position for the holding voltage (about 3 volts)and 'a second position for electroplating (about 6 /2 volts). Connectedelectrically to the rectifier is a timer which automatically establishesthe plating cycle. At all times except during the plating cycle apotential of between 2.8 and 3 volts is maintained between theelectrodes. The vibrator is preferably connected to the time so that itis operated throughout the plating cycle only and automatically shutsoff when the cycle is completed. The timer mechanism can be set to therequired time to obtain the desired coating thickness. The timer can bearranged to activate the plating cycle (by increasing the appliedvoltage) and the vibrator can be adjusted empirically to obtain thedesired optimum amplitude and frequency. As an example, an optimumfrequency range for zinc-coated wire of the type described above is 3000to 4500 c.p.m. with an amplitude range of 0.1 to 0.3-inch. Aftercomplettion of the plating cycle, the vibrator is automaticallydisconnected and the system reverts from the operating voltage to theholding voltage. In this way, the workpiece can be allowed to remain inthe electrolyte for prolonged times without appreciably changing thethickness or quality of the coating.

Any suitable electroplating unit may be used; however, a preferredsystem is a portable unit with a tank for electrolyte and adapted toreceive small sections such as the welded joint of coated wire strands.A suitable anode, such as a lead or silver-bearing-lead anode, ispositioned within the electrolyte tank so that the wire, clamped to thevibrator, may be inserted into the electrolyte with the area of the workto be plated centered over the anode. Electrode spacing of 1 to 3 inchesis desirable. It is also desirable that the workpiece not rest on theanode holder since this will dampen the vibration. A zinc sulfateelectrolyte is used for zinc coating which contains about 10 to 14ounces per gallon of 96% by weight H 80 ,v and about 22 to 34 ounces pergallon of zinc sulfate (ZnSO; H 0) electrolyte without a positivecurrent flow instantaneous from the anode to the workpiece, the metalcoating on the work would begin dissolving and form a loose deposit onthe strand surface. For example, it has been found that if a partiallyzinc-coated work is immersed into the electrolyte for as little as onesecond without a holding voltage applied thereto, the subsequent coatingis spongy and non-adherent.

It is apparent from the above that various changes and modifications maybe made without departing from the invention; For example, Where a timeris used as in the preferred embodiment, they device can be set toincrease the voltage automatically to the desired value for apredetermined plating period. After completion of plating, the timer canbe used to cause the voltage to be reduced to the holding voltage value.With this arrangement, it is not necessary for an operator to maintainthe system under surveillance at all times. Instead, the operator can befree to discharge other duties after the electroplating unit has beenset.

We claim 1. A method of electroplating a partially-coated metalworkpiece in an acid electrolyte containing a salt of the metal to beplated ,onto the work, comprising the steps of applying :a holdingvoltage potential to the partiallycoated workpiece prior toelectroplating which is above the decomposition potential of saidelectrolyte, contacting said workpiece with said electrolyte toelectroplate same and increasing the voltage applied thereto above saidholding voltage to increase the electroplating rate, maintaining saidincreased voltage for a period of time corresponding, to the desiredamount of plating to beaccomplished, reducing the voltage applied to theamount of said holding voltage and maintaining said holding voltage. onsaid workpiece until withdrawing same from said electrolyte. v

2. A method according to claim 1 wherein said workpiece is vibratedduring electroplating thereof.

.3. A method, of electroplating a partially Zinc-coated steel workpiecein an 'acid electrolyte containing a zinc salt comp'rising the steps 0fapplyinga holding voltage potentialto the workpiece prior toelectroplating which is above the decomposition potential of the zincsalt electrolyte, suspending the workpiece as a cathode in saidelectrolyte to electroplate same, increasing the" voltage a'niountofplating to be accomplished, reducing the voltage applied to"th e amountof the holding voltage upon completion of the plating cycle andmaintaining said holding voltage on said workpiece until it is withdrawnfrom said electrolyte.

4. A method according to claim 3 wherein a holding voltage of between2.8 and 3.0 volts is applied to the metal workpiece.

5. A method according to claim 4 wherein the voltage is increased tofrom about 6 to about 7.5 volts to electroplate same.

6. A method according to claim 3 wherein the Workpiece is vibratedduring electroplating thereof.

7. The method of claim 3 wherein said holding voltage potential to theworkpiece prior to contacting it with electrolyte is from 0.2 to 0.5volt above the theoretical decomposition potential of the metal saltmeasured in a 1 N solution at room temperature using platinumelectrodes.

References Cited UNITED STATES PATENTS 1,942,356 1/1934 Fink et 'al.20429 2,453,668 11/1948 Marisic et a1. 204-40 XR 2,744,860 5/1956 Rines20445 2,951,018 8/1960 Seaborg et a1. 204228 XR 3,336,658 8/1967 Husni204-45 XR JOHN H. MACK, Primary Examiner. G. KAPLAN, Assistant Examiner.

1. A METHOD OF ELECTROPLATING A PARTIALLY-COATED METAL WORKPIECE IN ANACID ELECTROLYTE CONTAINING A SALT OF THE METAL TO BE PLATED ONTO THEWORK, COMPRISING THE STEPS OF APPLYING A HOLDING VOLTAGE POTENTIAL TOTHE PARTIALLYCOATED WORKPIECE PRIOR TO ELECTROPLATING WHICH IS ABOVE THEDECOMPOSITION POTENTIAL OF SAID ELECTROLYTE, CONTACTING SAID WORKPIECEWITH SAID ELECTROLYTE TO ELECTROPLATE SAME AND INCREASING THE VOLTAGEAPPLIED THERETO ABOVE SAID HOLDING VOLTAGE TO INCREASE THEELECTROPLATING RATE, MAINTAINING SAID INCREASED VOLTAGE FOR A PERIOD OFTIME CORRESPONDING TO THE DESIRED AMOUNT OF PLATING TO BE ACCOMPLISHED,REDUCING THE VOLTAGE APPLIED TO THE AMOUNT OF SAID HOLDING VOLTAGE ANDMAINTAINING SAID HOLD VOLTAGE OF SAID WORKPIECE UNTIL WITHDRAWING SAMEFROM SAID ELECTROLYTE.